Law of identity (reflexivity of class equality). Theorem 6.4 of [Quine]
p. 41.

This law is thought to have originated with Aristotle
(Metaphysics,
Zeta, 17, 1041 a, 10-20: "Therefore, inquiring why a thing is
itself,
it's inquiring nothing; ... saying that the thing is itself constitutes
the sole reasoning and the sole cause, in every case, to the question of
why the man is man or the musician musician."). (Thanks to Stefan
Allan
and Benoît Jubin for this information.) (Contributed by NM,
5-Aug-1993.) (Revised by Benoît Jubin, 14-Oct-2017.)